DC motors and their supporting servo loops are widely used in applications that require driving a load having a high kinetic energy. One of the many uses of such motors is disclosed in U.S. Pat. No. 5,268,708, which describes an image processing apparatus arranged to form an intended image on a receiver secured to the periphery of an imaging drum while the drum is rotated past a printhead. In the device disclosed by U.S. Pat. No. 5,268,708, the imaging drum secures donor and receiver materials to its surface using a vacuum. In order to meet requirements for throughput productivity, the imaging drum rotates at high speed, moving the secured donor and receiver materials past a printhead that writes the image.
Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. The imaging drum, which presents a high inertial load when spinning at 600 RPM or more, must be regularly stopped to allow unloading and reloading of donor and receiver materials. This stopping action must be repeated four times for each four-color proof sheet output, since the process to create each color separation requires loading and unloading a different donor sheet. With the current image processing apparatus, no active braking mechanism for the drum is applied; drum rotational speed is therefore constrained to an upper limit of approximately 600 RPM by the need to coast the drum to a stop between color separations. This also extends the amount of time required between color separations, reducing throughput of the device.
Drum braking would also be advantageous when accelerating the drum. The high inertial load presented by the drum complicates the task of acceleration to a high velocity due to overshoot, where the drum spins faster than required following acceleration. To allow quick acceleration, it would be advantageous to be able to apply some amount of braking to compensate for overshoot.
Braking a motor that drives a load of high kinetic energy requires dissipation of this energy in some manner. For example, in an imaging processing system such as that disclosed in U.S. Pat. No. 5,268,708, with its drum rotating at a high speed of 3,000 RPM, the kinetic energy of the rotating system is in the range of 1750 Watt-seconds. To brake the motor driving this drum requires that its kinetic energy be dissipated over some interval of time. The dissipation wattage for braking this drum in exactly 5 seconds would be computed as the following ratio: EQU 1750 Watt-seconds/5 seconds=350 Watts
To be able to repeatedly stop the imaging drum within 5 seconds, the apparatus would be required to dissipate this amount of energy at regular intervals. This braking action can generate significant amounts of heat that must be removed efficiently from the image processing apparatus. At the same time, it is advantageous for this application to provide a controlled stop, in which the rotational position of the drum is known, to allow efficient loading and unloading of the imaging media.
There are a number of well-known methods for braking a DC motor and dissipating the kinetic energy of the motor and its load. Among these are mechanical methods that use friction, where brake pads or similar stopping devices are employed. However, mechanical braking introduces problems of wear, mechanical complexity, dust, and reliability. In addition, mechanical braking requires replacement parts and procedures, particularly for equipment having high inertial loads, such as the imaging drum of the type discussed above.
Conventional electrical braking methods for stopping a motor quickly include dynamic braking. This method works by reversing the flow of current in the motor armature while, at the same time, maintaining the motor field. This action effectively converts the rotating energy of the motor into current flow, so that the motor acts as a generator, producing a back-EMF current flow in reverse direction from the flow of drive current. A high-wattage braking resistor is then switched across the armature to dissipate this regenerated current, bringing the motor to a stop. The effective stopping speed is a function of resistance; the lower the resistor value (therefore, the greater the reversed current flowing through the armature), the faster the motor can be stopped. (Reference: Siskind, Charles S., Electrical Machines, Direct and Alternating Current, 2.sup.nd edition, McGraw-Hill, 1959, page 210.) At the same time, the resistor value must also be high enough so that it limits the current flowing through the armature.
By itself, dynamic braking does not provide a means for controlling position while stopping the motor. There would be advantages to using dynamic braking within a servo loop that provides position control at all times during motor acceleration, steady-speed rotation, and deceleration.
Dynamic braking is effective when the motor rotates above a threshold RPM; when motor RPM drops to a low value, field collapse causes dynamic braking to be less effective. When large motors are used, this type of dynamic braking typically requires heavy-duty relays or contactors for switching current across the load resistor. Conventionally, this resistive load is positioned at or near the motor terminals.
A problem inherent to electrical dynamic braking is heat dissipation. Conventionally, dynamic braking is not considered as suitable for applications requiring frequent start-stop cycling because of the high amounts of heat generated by dissipation across a load resistor. (Reference: R. L. McIntyre, Electric Motor Control Fundamentals, McGraw-Hill, Inc., 1974, page 31.) Proposed solutions for frequent braking in applications with high inertial loads suggest using one or more high-wattage resistors switched by means of solid-state devices, but these solutions are not suited for the high wattage dissipation required for the image processing apparatus noted above; moreover, solutions that simply substitute solid state devices for relays are not well-suited to high-wattage applications, particularly where such solid state devices are required to operate over their linear region (where heat generation in the solid state device can quickly destroy the device).
Among numerous patents that disclose methods and devices for electronic braking are the following:
U.S. Pat. No. 4,223,855 (Briedis) describes an electromechanical braking system that applies friction pads against rotating shafts for stopping a reel-to-reel tape transport. As noted in the background description of this patent, friction wear degrades the pads over time and causes continually changing brake characteristics. To minimize this problem, the patent discloses the use of a combination of dynamic and mechanical (friction pad) braking.
U.S. Pat. No. 4,911,566 (Imaseki, et al.) discloses a braking control system for a thermal printhead that shortens braking time by using a combination of dynamic braking and negative-phase braking. This method first short-circuits motor feeder-terminals (a form of dynamic braking), then applies a reversed-polarity voltage to stop the motor. Notably, this method is used with a small motor and low inertial load, where the load is substantially smaller than that presented by the rotation of the imaging drum as noted above.
U.S. Pat. No. 3,845,366 (Metzler, et al.) discloses a constant-torque braking control system for a system utilizing multiple DC motors, such as a printing press. With this system, braking resistors are switched across the armatures of the motors to dissipate the current that results from generator action as the motors are slowed. This method provides effective motor braking, but sacrifices position control. Using this method allows the motors to be stopped; but this invention discloses no method to slow the motors to compensate for overshoot or to change motor speed.
U.S. Pat. No. 5,659,231 (Svarovsky, et al.) discloses braking control circuitry for a DC brushless motor that first stops drive current to the motor to set it coasting, then sets a voltage level to oppose a feedback signal, forcing regenerated current flow to a controlled level for dissipation in the power supply regulator circuitry of the motor controller. Notably, this method dissipates power in the motor controller itself, limiting the regenerated current below potentially damaging amounts by using control circuitry as a "valve" to restrict current feedback. Conventional motor controllers are capable of dissipating some current, but are not capable of providing dynamic braking for the type of high inertial load presented by a rotating drum in an image processing apparatus.
A secondary problem for motor braking occurs in the event of power loss. This condition can be destructive or hazardous for motor driven applications. For an image processing apparatus such as the one cited above, loss of power while the drum is at full speed rotation and is loaded with donor and receiver material that are secured to the drum under vacuum, can mean fly-off of the media and consequent damage to the optical print head and to other sensitive support components. For this reason, fast and efficient braking of the DC motor upon power loss presents important advantages for imaging systems and other types of equipment.
Dynamic braking requires that the field energy to the motor be maintained. For this reason, conventional electronic motor brakes do not function if power is lost or disconnected. (Reference: Miller, Rex and Miller, Mark R., Electric Motor Controls, Prentice-Hall, 1992, page 214.) However, a number of methods have been disclosed for providing sufficient field energy to allow at least some level of dynamic braking in the event of power loss. For example, U.S. Pat. No. 5,099,184 (Hornung, et al.) discloses circuitry for applying sufficient magnetic field for dynamic braking, for a time, in the event of power loss.
A number of patents disclose the use of the regenerative energy of the motor as it rotates with a reversed armature current (hence, acts as a generator) when slowed using dynamic braking methods. Examples include U.S. Pat. No. 5,659,231, cited above, as well as U.S. Pat. No. 4,678,980 (Sugimoto, et al.) and U.S. Pat. No. 4,445,167 (Okado, et al.) in which regenerative motor energy provides temporary source power for motor control circuitry.
While known practices, references, and patents such as those cited above disclose various ways to implement dynamic braking, there are significant limitations and shortcomings to conventional methods. Systems such as the image processing apparatus cited above present these special requirements for motor braking:
Position control. Conventional methods for dynamic braking do not provide position control when the motor is stopped. PA1 Overshoot compensation. The methods described above disclose ways to stop a motor, rather than provide a means for slowing rotation momentarily to correct overshoot. PA1 Fast acceleration and deceleration for a high inertial load. This requires dissipating high levels of current. Conventional commercial motor controllers provide some level of capability to dissipate regenerated motor energy as current. However, these solid-state devices are not designed to dissipate the high levels of energy required for inertial loads such as presented by the imaging drum noted above. PA1 Frequent repeatability for a system having a high inertial load.
None of the patents and text references cited above disclose or suggest a suitable method for dynamic braking that varies the duty cycle of high levels of regenerated current across a dissipative load. Also, none of the patents or texts cited above suggest using such a method that operates both when controlled by machine commands and when power is lost. Further, these known methods do not implement dynamic braking within a servo loop. Additionally, these methods do not present a suitable process for switching a load across motor terminals using solid-state technology, given the high-wattage dissipation required for stopping an imaging drum.